Cyclobutyl pyrimidine dimers are produced in DNA by ultraviolet light (220 less than lambda less than 300 nm). Dimers are responsible for mutations and death in procaryotes and eucaryotes and are thought responsible for the induction of cancer by ultraviolet light. The photoreactivating enzyme is a repair enzyme specific for pyrimidine dimers in DNA. The enzyme binds to a dimer-containing region of DNA and on absorption of a near UV or visible photon (300 less than lambda less than 500 nm) monomerizes the dimer and restores the physical and biological integrity of the DNA. There are two physical mechanisms which might be operative in the action of the enzyme: energy transfer and formation of a molecular complex. Both of these possible mechanisms are consistent with known nonenzymatic physical mechanisms of monomerization. Measurements of the optical absorption and emission spectra of the enzyme in the absence and presence of UV-irradiated DNA will be used to distinguish between these tAo general mechanisms of photoreactivation and also between alternate paths possible for each general mechanism. The optical techniques of natural and magnetic circular dichroism, flash photolysis, and two new spectroscopic techniques, natural and magnetic circularly polarized luminescence (the emission analogs of CD and MCD) will provide more detailed information about the mechanism of monomerization and help determine suitable model systems. Such model systems will be studied by other spectroscopic techniques such as nuclear magnetic resonance, chemically induced dynamic nuclear polarization and infrared spectroscopy which could not be used effectively in a system containing enzyme and DNA because of low concentrations, long relaxation time and overlaping absorption bands. These experiments should reveal the mechanism of action of the photoreactivating enzyme and perhaps make feasible the selective introduction or removal of other DNA lesions.